(1) Field of the Invention
The present invention relates to an electrophotographic copying machine comprising a photosensitive member for carrying a latent image of an original illuminated by a light source, and a developing device for developing the latent image on the photosensitive member into a visible image.
(2) Description of the Prior Art
The electrophotographic copying machine noted above is also known as the PPC (plain paper copying) machine and is in wide use for recording clear images on ordinary recording paper speedily. One application of this electrophotographic copying machine is found in the production of an original for use in preparing a master plate for thermal stencil printing which is one type of stencil printing.
The master plate for a thermal type stencil printing machine is prepared by using the original and a stencil paper comprising a thermoplastic resin film affixed to a meshed support member. The stencil paper is pressed on the original such that the film is placed in contact with an image-carrying face of the original, and then the combination is illuminated at the stencil paper side by a light full of infrared rays. At this time the image portion of the original absorbs the energy of the light and generates heat, which heats the thermoplastic resin film in pressure contact with the original. The film then becomes fused and shrinks, forming holes in a portion of the film opposed to the image portion of the original, thereby producing the master plate.
When preparing the master plate for the thermal type stencil printing machine by using a colored starting document as distinct from the abovementioned "original" produced by the electrophotographic copying machine, this starting document is copied by the electrophotographic copying machine for use as the original for preparing the master plate. In other words, the colored document cannot be used as it is as the original for preparing the master plate because of its variation in hue which results in differing rates of light absorption and hence uneven heat generation. Therefore, the colored document is copied by the electrophotographic copy machine to eliminate its colors (the resulting copy being hereinafter referred to as PPC original) for preparation of the master plate.
Incidentally, it is difficult to make out copied letters and pictures when the density of a copied image is too low. In order to avoid this, the electrophotographic copying machine normally is set such that a solid pattern density as measured with a densitometer is at least 1.1, preferably not below 1.25. Furthermore, there is a known electrophotographic copying machine capable of varying the density of the copied image within the above-noted range in order to effect reproduction according to the density of the original to be copied.
However, the above known machine has the following problem and there is room for improvement.
With the electrophotographic copying machine, copies are produced by causing thermally fusible toner particles to electrostatically and selectively adhere to a latent image on the photosensitive member and thermally fusing these toner particles transferred onto recording paper to fix the particles thereto. However, the resulting PPC original also carries toner particles not sufficiently fixed to the image portion. Such toner particles tend to be the greater in amount the higher the density of the copied image is.
Consequently, when such a PPC original is used to prepare the master plate, the toner particles not sufficiently fixed to the original are melted by the energy of illumination which results in plugging of holes formed in the thermoplastic resin film. Prints produced by using this master plate, therefore, have a blurred image portion as a result of insufficient passage of ink through the holes of the film.
In order to eliminate such an inconvenience, the density of the PPC original may be reduced. However, with the known electrophotographic copying machine, the density remains in a range suited for normal copying even if the machine is set for the lower limit of the density of a recorded image wherein the low density suited for preparation of the master plate for the thermal type stencil printing machine cannot be realized.